The present invention relates to a process and an apparatus for cooling a hot reaction gas to a predetermined inlet temperature for introduction into a separator, especially as stage in manufacturing phthalic anhydride.
The starting point of the present invention is the preparation of phthalic anhydride (PAA) by oxidation of o-xylene. Hot o-xylene and hot air are reacted in the presence of a catalyst. The PAA thus formed leaves the reactor mixed with air as hot reaction gas and has a temperature of about 360xc2x0 C. In PAA plants with downstream reactors this temperature is a bit lower. The separation into crude PAA and air is then carried out in a downstream separator. Before entry into the separator(s), it is necessary to cool the hot reaction gas to a predetermined desired temperature, as a rule from about 160 to 175xc2x0 C.
Usually, a complicated, at least two-stage gas cooler having at least two steam drums and corresponding cooling means is used for this purpose. This makes the process which is used for working up the reaction gas and is downstream of the actual reaction complicated and expensive.
A combination of a main reactor with a downstream reactor with or without cooling means therein has been described already; examples are the DE-A 197 42 821 and DE-A 198 07 018.
It is an object of the present invention to provide an optimized process for cooling a hot reaction gas which permits a simpler and more economical design of a reaction system with lower maintenance costs and which results in a qualitatively good andxe2x80x94based on the produced amountsxe2x80x94quantitative reaction of the educts.
We have found that this object is achieved by a process for manufacturing phthalic anhydride (PAA) by oxidation of o-xylene in the presence of a catalyst, optionally in two steps in a main and a downstream reactor, in which the hot reaction gas from the main reactor comprising o-xylene and air is cooled to a predetermined inlet temperature for introduction into a separator, wherein a heat exchanger through which a cooling medium flows is located upstream of the separator, and the hot reaction gas is passed through the heat exchanger for interaction with the cooling medium.
The heat exchanger replaces the gas cooler, with the result that the total reaction system acquires a simplified design and lower costs are to be expected.
In an embodiment of the novel process, the inlet temperature of the reaction gas on introduction into the heat exchanger is from 250 to 400xc2x0 C., in PAA plants without downstream reactor preferably from 350 to 380xc2x0 C. and in PAA plants with a downstream reactor preferably from 280 to 320xc2x0 C., and its outlet temperature on emerging from the heat exchanger is from 130 to 180xc2x0 C., preferably about 160xc2x0 C. However, the heat exchanger is also capable of cooling the reaction gases or other gases at other predetermined inlet temperatures to said outlet temperature of the heat exchanger or alternatively to another outlet temperature.
It is particularly advantageous that the cooling of the reaction gas to the predetermined inlet temperature for introduction into the separator can be effected in one stage in the heat exchanger by the novel process.
According to a preferred embodiment of the novel process, a gaseous cooling medium, particularly preferably air, is used.
The inlet temperature of the cooling medium, e.g. air, on introduction into the heat exchanger may be below 100xc2x0 C. and its outlet temperature on emerging from the heat exchanger may be from 300 to 350xc2x0 C., preferably about 330xc2x0 C. This results in a particular advantage of the novel process, whereby the cooling medium heated by the interaction with the reaction gas is in turn fed into the reactor as one of the reactants for the chemical reaction. The temperature of the cooling medium emerging from the heat exchanger is in fact, according to the novel process, about 150xc2x0 C. higher than is usual in processes known from the prior art. Consequently, it has a temperature which permits direct introduction of the cooling medium into the reactor. The coolant is chosen so that it simultaneously constitutes one of the reactants of the chemical reaction.
In another embodiment part of the cooling medium is led via a by-pass around the heat exchanger and is again mixed with the heated cooling medium leaving the heat exchanger. Based on this a varying temperature of the cooling medium preferably used again in the reaction is possible. Preferably such a modification is realized by the temperature steering of the amounts passing the by-pass.
If the chemical reaction serves for preparing PAA, hot o-xylene, as one reactant, is reacted in the presence of a catalyst and hot air as a further reactant. The hot reaction gas formed as a result of the reaction essentially comprises PAA and air. Since air is simultaneously the cooling medium for the hot reaction gas andxe2x80x94as indicated further abovexe2x80x94emerges from the heat exchanger at a considerably higher temperature than is possible in the prior art, it can be fed directly into the reactor. A further advantage is that the o-xylene initially having a temperature of about 30xc2x0 C. is fed into the line containing the hot air and is simultaneously also heated without further apparatus and energy costs. Nevertheless, the result is an inlet temperature into the only or into the main reactor which is about 100xc2x0 C. higher compared with the prior art, which leads to better utilization of the catalyst and a smaller heat-up zone in the reactor.
In contrast, it has been necessary to date in the prior art to preheat the air in an inconvenient and energy-intensive manner by means of a preheater. The liquid o-xylene preheated in a separate steam-heated heat exchanger was fed into this preheated air. However, the mixture of air and o-xylene did not reach the reactor inlet temperature achieved by the novel process. Moreover, it was found that more high-pressure steam can be generated in the reactor in the novel procedure.
Although the advantages of the novel process are presented with reference to the preparation of PAA and its advantages are described in comparison with the process known from the prior art for the preparation of PAA, the use of the apparatus necessary for cooling a hot reaction gas is not restricted to this chemical reaction. It can be used very generally for cooling hot reaction gases and can be employed for any desired chemical reaction which is carried out in a reactor at elevated temperatures and in which the cooling medium of the reaction gas emerging from the reactor, in the heat exchanger, is simultaneously required as a reactant or solvent (liquid or gaseous) for carrying out the chemical reaction.
A further achievement is an apparatus for cooling hot reaction gases to a predetermined inlet temperature for introduction into a separator after a foregoing reaction in a main reactor and optionally a following downstream reactor, comprising
(a) a means for conducting the main reaction,
(b) a means for heat exchanging,
(c) optionally a downstream reactor before the means (b) and optionally having a means for intermediate cooling,
(d) a means for separating a reaction product
and connections to and between the means.
The heat exchanger used can be any desired heat exchanger, for example a tubular beat exchanger. A gas/gas plate-type heat exchanger is preferably used.